The present invention relates in general to conditioning of seed cotton for ginning, and specifically for drying of high moisture seed cotton, and more particularly to seed cotton drying systems involving a tower dryer wherein seed cotton descends along a continuous restricted zigzag path defined by shelf-like partitions in a casing forming the tower dryer, with the seed cotton being impelled along the zigzag path by a high velocity stream of heated air.
Heretofore, the harvesting of high moisture seed cotton has been recognized as a quality problem as early as the 1930's and much work has been carried on in developing methods of drying the seed cotton prior to ginning the cotton, including work carried on by the U.S. Department of Agriculture Ginning Laboratory at Stonesville, Miss. Work by the U.S.D.A. Ginning Laboratory resulted in what has become known as the parallel flow tower dryer, which for many years has been the most prevalent type of seed cotton drying employed at the pre-ginning stage in cotton ginning installation.
Those early efforts were directed at lowering the moisture content of the seed cotton enough to allow the gin to produce a smooth sample. High moisture seed cotton resulted in rough preparation of the lint. The "preparation" of the lint was a very important quality factor, along with "Grade" and "Staple". More recently, the wide use of lint cleaners which involve a combing action, which are customarily used in gin plants in association with gin stands or between the gin stands and the battery condenser, result in smooth preparation due to the combing action of the lint cleaners under almost all moisture conditions and the "preparation" factor is no longer as significant a factor in quality as it once was.
Prior to World War II, a number of gins were equipped with dryers to improve the preparation, but in those cases, except for the arid areas of Texas and Oklahoma, very little seed cotton cleaning was used in addition to the drying. The labor shortage of the post-World War II period made the careful hand harvesting of seed cotton impractical, and mechanical harvesting of cotton developed at a very rapid rate. This usually resulted in the adding of moisture to improve the efficiency of the seed cotton harvester, resulting in a significant increase in the need for drying at the gin plant prior to the ginning of the seed cotton. Also, the mechanical harvesting method resulted in much more foreign matter being brought to the gin. It was found that drying the lint to a low moisture content improved the efficiency of the seed cotton cleaning achieved at the gin, and gins across the country rapidly installed two states of drying and began to use more and more heat energy to produce better grades of cotton notwithstanding the mass of leaf trash and stems in the high moisture seed cotton resulting from mechanical harvesting methods. In fact, use of three states of drying in gins to improve seed cotton cleaning is not uncommon today.
The parallel flow tower dryer has become the most commonly and successfully used of the drying apparatus by the ginning industry. The parallel flow tower dryer method involves the use of direct fired heaters usually rated at from 3,000 B.T.U. up using natural gas, liquid propane gas or oil, for heating the conveying air designed to impel the seed cotton through the tower dryer. This air for drying and impelling the seed cotton passes through the heater from a suitable blower, picks up the seed cotton to be dried from a rotary airlock under a feeder, and conveys it to the top of the parallel flow tower dryer. The tower dryer involves a plurality of parallel vertically spaced shelves which alternately extend from one end wall of a vertically elongated casing to a location near but spaced from the opposite end wall to define a zigzag or labyrinth path descending from the top to the bottom of the tower. The heated air conveys the cotton along the shelves of the tower, dropping it from one shelf to the next and tumbling the seed cotton as it changes directions at the end of the shelf. As the seed cotton reaches the bottom of the tower dryer, the conveying air carries it through a duct to an air-separating unit where the drying air is separated from the cotton and discharged to the atmosphere.
An important factor in the efficiency of the tower dryer system is the velocity of the air over the shelves, referred to commonly as "shelf velocity". Obviously, the shelf velocity has to be high enough to convey the seed cotton across the shelf and, unfortunately, this required velocity varies with the density of the cotton which is a function of moisture content. The best efficiency is obtained at the lowest shelf velocity which will convey the cotton along the shelves. In the early stages of development of the tower dryer, it was found that about 40 cu. ft. of air per pound of material was optimum. At this ratio, shelf velocities as low as 900 feet per minute were very successful. This resulted in low static pressure losses across the tower and only moderate air temperatures were required.
Of course, such tower dryer systems were expected to handle only damp cotton, varying in lint moisture from about 8% to 10%. As opposed to this condition, modern high capacity gin plants are set up to handle maximum moisture contents of from 15% to 20%, although only a small percentage of the cotton being dried has such a high moisture content. This requires shelf velocities in the range of about 3,000 feet per minute to successfully convey the cotton over the shelves at ratios of 25 to 30 cu. ft. of air per pound of material. At these velocities, the cotton is exposed to the drying air for a very short period. To compensate for this short exposure, higher temperatures are used, resulting in excessive consumption of fuel.
An object of the present invention is the provision of a tower dryer system which will reduce energy and fuel consumption and achieve higher efficiency drying of the seed cotton, thereby achieving significant economies in the initial cost of seed cotton drying systems because of reduced horsepower requirements for air propelling equipment and achieving economies in operation due to greater conservation of heat energy.
As a means of attaining this object, I have devised a parallel flow tower dryer which provides for a high shelf velocity over approximately the upper one-half or upper one-third of the shelves, which will lower the density of the seed cotton material being handled sufficiently to permit a reduced velocity over the next approximately one-half or one-third of the shelves. The reduced velocity in this lower one-half, or middle approximately one-third of the shelves is accomplished by a wider shelf spacing. In the case of three different shelf-spacings, the lower approximately one-third of the shelves has a still wider shelf spacing for a still lower velocity, since the seed cotton continues to become less dense as the air absorbs the moisture.
Further, I have attained greater heat conservation and heating efficiency in the system by increasing the shelf spacing at intervals of the downward travel sufficiently to accommodate a fresh supply of heated air between the shelves where there is an increase in spacing. One innate weakness of the present tower dryer structure is the rapid cooling of the drying air as it absorbs moisture. When high moisture cotton is introduced into the drying and conveying air, it is rapidly cooled by evaporation, as well as radiation losses. In order to maintain an effective drying temperature at the separation point, the initial temperature must be very high, especially if the moisture content of the cotton is high.
By providing several stages of increased shelf spacing and applying fresh heated air between the shelves where there is an increase in shelf spacing, an even air temperature can be maintained throughout the drying system. The source of heated air to be applied between the shelves can be the same heating source as that used for heating the conveying air, or a separate source of heat may be provided. By keeping the temperature of the conveying air up to a point which will provide a good moisture transfer rate throughout the system, the efficiency is greatly improved, and an efficient moisture transfer can be accomplished by such an even heat dryer construction without having excessively high temperatures at the inlet of the drying system.
Yet another object of the present invention is the provision of a novel even heat parallel flow tower dryer structure for drying seed cotton, wherein horizontal shelves extend in alternation from opposite sides of the tower to locations near but spaced from the opposite side to define a continuous zig-zag cotton flow path from the top to the bottom through which the cotton travels impelled by conveying heated air, and wherein a relatively higher shelf velocity is provided for in the upper half of the dryer, while a selectively reduced shelf velocity is provided in the lower half by increasing the spacing of the shelves, or wherein three or more different shelf spacings are provided with the uppermost shelf spacing being smaller and the succeeding lower sections having a larger spacing than the preceding section, thereby selectively reducing shelf velocity in the successive sections of different shelf spacing, and wherein fresh hot air is supplied at each increase in shelf spacing to maintain a more uniform rate of moisture absorption through the height of the tower dryer.
Other objects, advantages and capabilities of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings illustrating a preferred embodiment of the invention.